NEW YORK (CNNMoney.com) -- Car makers are confident they can meet new government rules calling for a national fleet average of 35 miles per gallon. But it will take a big technological push, they say.

You might wonder why, since twenty years ago the car that got the best mileage in the nation was a real techno-wimp compared to what's on the road today. It wasn't even a hybrid. But it got better fuel economy than any car sold now - even the Toyota Prius.

Looking back at the 1987 Honda Civic CRX shows us why cars use so much more gas today and about the trade-offs we've had to make.

One answer for the mileage drop is that the rating system has changed. Beginning with the 2008 model year, the EPA began using a more rigorous fuel economy test that means lower numbers for most cars. But that's only a small part of the answer.

If the old CRX HF were tested using today's rules, its highway fuel economy would drop to 51 mpg, according to the EPA's calculations. That's still much better than any mass-market car sold today, including hybrid cars.

The bigger answer is that the Honda Civic has changed a lot in twenty years. Honda no longer sells a tiny two-seat version like the CRX. Even Civics with back seats are much bigger and heavier today than similar versions were in 1987.

It's in the nature of the car business that companies want to offer more - more legroom, more trunk space - with each redesign. As a result, cars get bigger and bigger.

Besides size, American consumers expect a lot more convenience out of a car than they did in 1985. Today, we expect power steering, power brakes, power windows and more.

The base CRX HF did not have power steering or power brakes. (As light as it was, it really didn't need them.) Air conditioning was optional, as it was on most cars in those days, so it didn't figure into the EPA's fuel economy ratings.

Today's consumers also expect safety. In the 1980s, car companies would sell cars that got one-star or two-star crash test ratings. Numbers like that would now cause car companies fits. Four out of five stars is considered the minimum acceptable rating.

The modern Civic has airbags front and side, electronic stability control and built-in crash protecting structures in the body. (See correction.)

Even the CRX's biggest fans wouldn't relish the thought of getting into a wreck in one of those cars. While actual crash test results are not available, even a Honda (HMC) spokesman admitted the car probably wouldn't have fared well by modern standards.

"Without the benefit of modern crash structure and extensive use of high strength steel, cars from two decades ago couldn't match the crash test performance of today's Hondas," said Honda spokesman Chris Naughton.

"It's kind of a classic engineering fight where safe cars compete with more fuel-efficient cars," said Todd Lassa, a writer for Motor Trend magazine and a CRX aficionado.

Lassa once owned a CRX DX, one step up in price and performance - and down in fuel economy - from the HF. (A 1987 sales brochure he still has provided some of the numbers for this story.)

A fun car to drive

Not that the CRX was a bad car. Far from it. Even before Honda introduced a performance version called the CRX Si, the lightweight, fun to drive Civic CRX was Motor Trend's "Import Car of the Year" when it first hit the market in 1985.

Even in its base HF trim, the CRX was considered a fun car to drive because it was small and responsive. Its zero-to-sixty time, though - about 12 seconds by some estimates - would put it well behind even a large, sedate family sedan like the Ford Taurus today.

Weighing less than 1,800 pounds, the CRX HF was powered by a 58-horsepower engine. Today's base Honda Civic weighs almost 2,600 pounds and is powered by a 140 horsepower engine. That's about 12.5 pounds less weight per pony today, despite greatly increased size.

"The lightest cars you can buy today are about 40 percent heavier than that car," Lassa said of his old CRX.

Daimler is about to find out how much appetite American's now have for inexpensive little two-seat cars that emphasize fuel economy over performance. It's just begun selling the tiny Smart ForTwo here. But even the ForTwo, which is smaller than the CRX, will get about 41 mpg on the highway, according to Daimler. (Official EPA estimates aren't out yet.).

Rumors swirl today, as they have for years, that Honda is planning to bring out a modern version of the CRX. Lassa says he pushes the idea whenever he speaks with Honda executives.

This time, though, the CRX HF would have to be a hybrid, he said. (Perhaps the one the company just announced it will make for 2009.) There just isn't any other way to pull that off today.

Correction: An earlier version of this story mentioned that the Honda Civic was an Insurance Institute for Highway Safety Top Safety Pick. In fact, it the Civic was removed from that list when the Insurance Institute added a requirement for electronic stability control, which the Civic does not have. (Back to story.)